The present invention is directed to the removal of inks from recycled paper products, i.e. deinking. "Deinking" is the process of removing ink and other contaminants from waste paper and there are two main techniques in current use. "Flotation deinking" entails forming an aqueous suspension of waste paper pulp fibers, ink, and other non-cellulosic contaminants and then mixing air into the suspension. In the presence of certain additives, air bubbles selectively attach to ink particles and carry those particles to the surface of the aqueous suspension, thereby forming an ink rich froth. The froth is then removed leaving behind a relatively ink-free fiber slurry. Flotation deinking processes are especially useful in removing hydrophobic inks with particle sizes larger than about 10 .mu.m. The additives used in such processes are generally specialty surfactants or fatty acids which are intended to agglomerate the relatively finer ink particles to increase removal efficiency in the flotation stage. The presence of additives which would disperse the ink particles rather than agglomerate them is considered detrimental to the effectiveness of the flotation stage.
"Wash deinking", on the other hand, is particularly useful when the ink and other particles being removed are finely dispersed so that the particles are smaller than about 5 .mu.m. This requires the addition of dispersants so that when a dilute waste paper pulp slurry is thickened, the fine flexographic type inks will tend to stay with the water being removed. Thus a relatively clean pulp is produced.
Newspapers, magazines and other printed media have been recycled for many years. Recently the need to recycle paper has increased significantly and will likely continue to increase in the future in view of environmental concern and legislative action. To reclaim fibers from printed material, a deinking process is required to remove the ink and other contaminants. Deinking of waste paper has become increasingly more difficult, however, because of changes in the printing techniques being used and the wide variety of printing inks. As a result, a slurry of recycled waste paper contain mixtures of inks, resin binders, fillers, and the like, which must be removed. The materials being removed have a wide range of particle sizes. The use of polymeric binders such as polystyrene, polyacrylates, and acrylic copolymers, has made removal of many of the newer inks more difficult or impossible in a wash-only deinking process. The industry response has generally been to utilize deinking systems which combine both flotation and washing processes.
However, the requirements for flotation and wash processes are different and are in conflict with each other. In flotation processes the ink particles are agglomerated to a size where they can be removed while in washing processes the ink particles are reduced in size and highly dispersed so that they can be removed.
This dichotomy has been further effected by recent changes in printing techniques which have resulted in an increasing usage of hydrophilic flexographic inks. The very small ink particles (&lt;5 .mu.m) associated with such flexographic inks are generally removed by means of a washing process. However, the very fine particle size of flexographic inks and the increasing use thereof has caused increasing problems in attaining the brightness necessary to enable the use of recycled paper products in many applications.
The conventional industry chemical formulations for flotation deinking for many years have included a fatty acid or fatty acid soap. See, for example, U.S. Pat. Nos. 4,964,949 and 4,483,741. However, fatty acids and fatty acid soaps have a number of problems associated with them, e.g. high dosage rates (typically about 16 pounds/ton of waste paper but as high as 30 pounds/ton), relatively poor foamability thus causing the high dosage rates, and the general need for high levels of water hardness to achieve acceptable performance. The hard water often leads to handling problems as well as scale and deposit buildup on mill equipment because of precipitation of the fatty acid soaps. More recently, non-ionic surfactants have been developed for use in deinking systems.
However, neither the fatty acids/fatty acid salts nor the non-ionic surfactants alone or in combination have been able to efficiently agglomerate the very fine hydrophilic flexographic ink particles sufficiently to allow their removal in a flotation unit operation. The problem is compounded for removal of hydrophilic flexographic inks because deinking processes are generally operated under alkaline pH, between about 8.5 and 10, while the hydrophilic flexographic inks become insoluble at pH's below about 7 so that neutral or acidic conditions are desired for their removal.
The use of various types of cationic materials in compositions for removal of inks from waste paper has been disclosed in several earlier patents. For example, polyoxyalkylene compounds containing amine or quaternary ammonium groups have been disclosed in such as U.S. Pat. Nos. 4,483,741, 4,605,773, JP 59 137587, DE 3,928,599, DE 4,007,598, and DE 4,007,597. Similar formulations which are claimed to be effective specifically for deinking of paper printed with flexographic inks have been disclosed in DE 4,007,596, WO 90 05806 and EP 478505. Surfactants such as the ones described in the preceding patents may be regarded as materials in which a single substance contains both non-ionic and cationic moieties. None of these prior art compositions, however, have been found capable of adequately removing the very fine hydrophilic inks in a flotation deinking stage or a combined flotation and wash deinking system.
Accordingly, it is an object of the present invention to provide a method of deinking printed media which contains very fine hydrophilic ink particles, e.g. the flexographic inks present in increasing quantities in waste paper, in a flotation deinking process.
It is a further object of the invention to remove the inks while minimizing the hardness of the water used in the aqueous slurry.
These and still further objects will be apparent from the ensuing description of the present invention.